This invention relates to the composition of the electrodes in an electrolytic moisture detection cell of the type employing a normally solid and regenerable hygroscopic electrolyte.
An electrolytic moisture detection cell generally involves two electrodes spaced apart from each other and in contact with a nonconductive substrate. A film of a hygroscopic electrolyte, generally phosporus pentoxide, fills the space between at least a portion of the electrodes. Often, it occupies all of the available space between the electrodes and also coats the electrodes themselves.
Upon the application of an electric potential, the electrolyte, in the absence of any moisture, does not permit current to flow between the electrodes. However, with water present, the electrolyte absorbs moisture; becomes conductive; and electrically bridges the space between adjacent portions of the electrodes.
As the current flows between the electrodes, the water electrolyzes to hydrogen and oxygen. The electrolyte thus continuously regenerates itself. Further, the electrical energy consumed in the electrolysis represents an accurate measure of the moisture absorbed in accordance with Faraday's law.
The composition of the electrodes represents an important consideration in the construction of an electrolytic moisture detection cell. Platinum and palladium wires have found the most frequent use in these electrodes.
J. W. Reeds, Jr., in his U.S. Pat. No. 3,223,609 of Dec. 14, 1965, teaches the use of rhodium or rhodium alloy electrodes wound helically on the inside surface of a cylindrical wall forming part of the cell container. Reeds suggests that using rhodium in these electrodes, or at least the anode, will avoid the development of black metallic deposits between adjacent turns of the two electrodes which previously caused shorting of the cells and ended their useful life. Reeds further recites that, after the electrolysis of water to H.sub.2 and O.sub.2, the rhodium also reduces the recombination of the hydrolysis products to reform water and give erroneously high results, particularly in gas streams having concentrations of H.sub.2 greater than 50%. According to the inventor, alloying the rhodium with any other metal, while producing a cell with a longer life that those totally lacking rhodium, nonetheless engenders inferior cells as compared to those with pure rhodium electrodes, or at least a pure rhodium anode. In particular, Reeds states that iridium represents one metal which, when alloyed with rhodium, produces inferior cell quality than pure rhodium.
Recently, however, electrolytic moisture detection cells have seen the development of a drastically new design for the electrode and electrolyte. This design, discussed below with reference to FIGS. 1, 1A, and 2 utilizes thin ribbon metal electrodes attached to and supported by a nonconductive substrate. This structure generally involves electrodes one micron or thinner. In comparison to the usual helically wound wire electrodes, the thin ribbon electrodes of this design do not possess their own mechanical rigidity. Rather, they must derive their stiffness from the substrate to which they adhere.
Should the thin ribbon electrodes become dislodged and lose contact with the substrate, loss of electrical contact with the hygroscopic electrolyte follows. The electrolytic cell then becomes inactive and useless at those locations where the electrodes have undergone such buckling. As a result, the cell suffers a reduction in its capacity to conduct current and accordingly saturates at abnormally low current levels.
Furthermore, in these inactive cell areas, the hygroscopic electrolyte nonetheless continues to absorb water. However, in order to undergo electrolysis, this moisture must now migrate to active portions of the cell where the electrodes maintain their adherence to the substrate. The period required for this migration represents a deleterious increase in the cell response times.
The problems associated with the loss of contact between the electrode and the substrate increase in severity in electrolytic diffusion cells. In these cells, discussed with regards to FIGS. 1 and 1A below, a diffusion barrier separates the sample undergoing analysis from the electrodes, electrolyte and their immediately adjacent area.
The diffusion cell incurs this increased severity in two regards. First, the diffusion barrier retards the excape of the H.sub.2 and O.sub.2 hydrolysis products which appears to aggrevate the electrode buckling. Second, the diffusion barrier retards the ingress of moisture from the sample and results in a cell response time already longer than for normal moisture cells; this increased response times necessitated by the diffusion of water from portions of the cell inactivated by buckled electrodes to active cell areas adds to the generally longer diffusion cell response times to give excessively and undesirably long overall response times.